Generally, conventional direction finders use antenna array to find the direction of the source of electromagnetic radiation. The direction of arrival is determined mainly by receiving the signal from the source via a plurality of antennas and comparing the intensity or phase of the signals.
A phase comparison direction finder determines the angle of arrival (AOA) from the phase difference of the signals received via antenna array, and is known to provide higher resolutions compared to an intensity comparison direction finder. In the ES system, where the AOA of received signal is obtained by comparison of phases, a direction finder scans frequency in the range from a few megahertz to tens of gigahertz to find direction which makes the construction of the system complicated and causing phase errors due to various factors.
FIG. 1 shows the construction of a conventional phase comparison direction finder.
As shown in FIG. 1, the conventional phase comparison direction finder comprises a signal receiver which receives signal through a single antenna 5, and a direction detector which detects direction through an antenna array 6.
In the above conventional phase comparison direction finder, phase error is generated by the front end receiver 10 and the direction finding (DF) down converter 50. Phase error in the front end receiver 10 is caused due to the arrangement error of each antenna and because each antenna is not exactly the same as one another both physically and electrically, and because the phase of electromagnetic wave is changed while transmitting a radome installed to protect the antenna, and also because of the phase difference between cables connecting the antenna and the switch. This type of phase error is corrected through radiation correction.
Also, the DF down converter 50, which generates phase error, is composed of various channels for evaluating phase difference among antennas in the antenna array 6, a plurality of RF devices and cables. Theoretically, phase error does not add to the phase difference when the electrical length of each channel is all the same, but the output includes the phase difference and the generated error due to the effect of a plurality of devices composing the down converter 50. This type of phase error, which is generated at the devices connected between the front end receiver 10 and digital phase detector 60, and at RF cables, is corrected through channel correction.
Generally, the ES equipment, which receives broad band frequency signals, generates phase error depending on the frequency and intensity of received signal, a receiving angle and internal temperature of the system. So measurement is made per each variable to be corrected after manufacturing the ES equipment, and the measured values are stored in the direction finding correction look-up table 85. The direction finding correction look-up table 85, as shown in FIG. 2 for a direction finder using four antennas, stores the values of radiation correction and channel correction per channel for correcting the error against the frequency and intensity of received signal, a receiving angle.
Also, the AOA generator 170 has the function of controlling BIT equipment 150, which only controls the generation of signals of the BIT equipment, and includes a phase detection data storing memory which temporarily stores the output of the phase detector. In this direction finding correction look-up table 85, the errors which are generated from the front end receiver 10 and DF down converter 50 respectively are stored.
Every time the ES system receives signals from the electromagnetic wave source, AOA is obtained after error has been removed by comparing the signal with the variable data of the look-up table stored according to the frequency, intensity and direction of received signal. The number of the antennas of the ES system or the devices of the DF down converter can be increased in order to obtain more accurate AOA. The increase in the number of the antennas of the ES system or the devices of the DF down converter, however, requires more DF correction look-up table, causing the problem of increasing the amount of memory of the correction look-up table.
Also, there is a problem of renewing the correction look-up table due to the corresponding change in the phase of a channel path whenever one of the channel devices or RF cables of a direction finder is broken and changed.